I think the staple of a community is that you can rely on each other. Commonalities alone don't make a community. Shared belief don't make a community either, only a cult. In a community people must be able to bring together multiple perspectives. In a community people must be able to have differences, and reconcile them through consensus. A community must be able to shine more lights on reality, not create an alternative realities.
HN has a semblance of a community, we strive to broaden our perspectives, and we welcome each other's entrepreneurial efforts.
I do like that hacker news does have a set of rules, and people try to enforce them (i.e. avoiding divisive topics, down voting low value comments, etc), but this alone a community does not make.
I'd consider HN and other online "communities" to be somewhere in between, where people form relationships because of some abstract notion of common constraints/goals/etc. Maybe there's a better term, though, to describe social groups that have something in common without everyone necessarily having direct contact with each other.
I think what's common to all social groups, though, is some level of trust. I come to HN because I trust the opinions and incentives are somewhat close to my own. So I'd consider it to be a community in that sense but it doesn't seem completely accurate since there's no direct contact between everyone.